The present invention relates in general to optical spectroscopy, and in particular, to a new and useful apparatus and method of determining the composition of fluids and material surfaces in a high temperature environment, particularly in a nuclear steam generator.
Various forms of denting, corrosion, and mechanical damage have been observed in both once-through and recirculating nuclear steam generators. Fortunately, the magnitude of the damage has not been extensive in most units; however, some units have experienced widespread damage. Any amount of damage is very expensive to correct and undesirable. The types of damage and the number of affected units are increasing.
Damage to steam generator components, such as tubes, tube support-plates, and tubesheets are often associated with deposits on or adjacent to the damaged component. Impurities in the feedwater can concentrate on the tubes and in adjacent crevices, forming corrosive aqueous films and solid, metallic oxide deposits. Metallic oxide formation can mechanically dent the tube, whereas, aqueous films can lead to various forms of corrosion. Additionally, the aqueous film can migrate through a porous oxide deposit and corrode the tube under the deposit.
Deposits originate from impurities in the water and steam, and from corrosion of components within the steam generator. Formation of deposits may be expected when:
1. The solubility of a chemical in the water or steam is exceeded,
2. A chemical dissolves in a water droplet and is transported to a location where the water droplet evaporates, or
3. A chemical in the water or steam reacts with a steam generator component to form another chemical.
Currently, water and steam samples are cooled to ambient temperature for analysis in the laboratory or by a continuous on-line monitor. Continuous monitors offer the following advantages over laboratory analysis:
1. A continuous or semi-continuous record of the water and steam chemistry,
2. Relatively small analytical time, and
3. Unattended operation at a significantly reduced labor intensity.
However, reduction of temperature and pressure prior to the analysis is a major disadvantage of both the laboratory analysis and the continuous monitors.
Reduction of temperature and pressure can alter chemical equilibria that are established at the higher temperatures. Chemical reactions and shifts in the concentrations of the various chemical compounds can result from the current practice of sampling water and steam from the steam generators. For example, ammonium sulfate, ammonium hydrogen sulfate, and sulfuric acid may be present in the high-temperature steam; however, only the total ammonium-ion and total sulfate-ion concentrations can be measured in a condensed ambient-temperature sample.
Furthermore, the chemistry of the deposits is also difficult to ascertain from the current sampling and analysis procedures.
Water evaporates from the aqueous deposits during the shutdown procedure, thus, only the non-aqueous constituents of the deposit remain after shutdown. Deposits are frequently scraped from the surfaces after shutdown for subsequent laboratory analysis to identify those constituents. However, the amount of water that was associated with the deposit during steam generator operations cannot be determined from the laboratory analysis. Therefore, deposit concentration cannot be determined by current sampling and analysis techniques. Additionally, some water soluble deposits may be washed away during the process of steam generator shutdown.
Post-damage analysis of steam generator tubes and deposits have provided most of the available information on the damage. Some preventive measures have been recommended after careful examination of the evidence. However, a timely method is needed to detect the formation of metallic oxides and corrosive chemical solutions within the steam generators before costly damage occurs. The analysis must be done at elevated temperatures and pressures within the steam generator, since evidence can be easily altered or washed away during shutdown. Further, the ability to make these measurements at several different locations within the steam generator is necessary, since several mechanisms are responsible for damage at various locations.
Spectroscopy, using a laser as the light source, has been disclosed in U.S. Pat. No. 3,463,591 to Franken, et al; U.S. Pat. No. 3,551,053 to Windsor, et al; and U.S. Pat. No. 4,645,342 to Tanimoto, et al. In these references, the light emitted by a laser is shined on a sample to be analyzed. The sample absorbs and reflects light in a manner which is characteristic of its composition.
None of these references, however, disclose the usefulness of optical fibers for conveying light into and out of a high temperature environment to analyze the composition of materials in the high temperature environment.
An article entitled "Remote Detection of Groundwater Contaminants Using Far-Ultraviolet Laser-Induced Fluorescence", by Chudyk, et al, ANALYTICAL CHEMISTRY, Volume 57, No. 7, June 1985, discloses a UV radiation source in the form of a nitrogen-pumping dye laser, for use in conjunction with optical fibers to analyze ground water containing various pollutants. The article discloses a test conducted with groundwater in a test tube. The optical fiber was coated with Teflon (a trade name).